Hydraulically tensioned nuts, washers and similar fasteners provide a means by which a stud or bolt can be tensioned by hydraulically actuating the nut or washer to exert a tensile force on the stud or bolt. These nuts and washers often operate under extreme pressure and temperature.
Hydraulic nuts or similar fasteners are typically pretensioned mechanically and thereafter hydraulic pressure is applied to a chamber within the fastened structure to generate an hydraulic force which applies an axial tensile load to a stud or nut engaged by the fastener. A locking collar may be provided to retain the tension after relieving the chamber of pressure.
Seals for use with hydraulic pressure devices are typically made of elastomeric material such as nitrile rubber or polyurethane. The ways in which these seal against the passage of fluid pressure can be divided into two types referred to herein as primary and secondary mechanisms. The primary mechanism of sealing acts during the initial application of fluid pressure. As this pressure increases, the elastomeric seal is deformed and forced into a position where the seal bridges the gap to be sealed, hereinafter referred to as the “extrusion gap”, in order to establish a secondary seal.
It is typical of hydraulically activated piston/cylinder arrangements that as the operating pressure increases, the cylinder walls expand radially causing a proportional increase in the extrusion gap between piston and cylinder. A limiting factor in the operation of hydraulic nuts is the effectiveness of their seals. Factors such as high pressures, high temperatures, service life under adverse conditions, limit their field of application and effectiveness. If these factors become extreme, either singularly or in combination, the materials which are commonly used as sealing agents fail. Failure occurs when there is flow or movement of the seal material into the extrusion gap under pressure and/or temperature and sealing is lost.
In extreme temperature/pressure applications, such as in electricity generators and nuclear power plant reactors, it is critical that seals do not fail as loss of tension applied to the studs or bolts for example in a generator housing or at a pipe flange joint, as such failure could result in a catastrophic disaster. U.S. Pat. No. 6,494,465 (Bucknell) (=International Application PCT/AU97/00425=International Publication WO 98/00660) discloses a range of hydraulic seals for hydraulic assemblies capable of operating at high temperatures. The seals incorporate lips which provide low pressure sealing between for example, a piston and a cylinder, and which are configured to move across the gap to be sealed at higher pressures with a base angled on a slope or a cup shape nestled into a groove. The seals may be formed of elastomeric material and/or thin sheet metal.
The seals of U.S. Pat. No. 6,494,465 have been used in many successful installations of high temperature, hydraulically tensioned fasteners in the electricity generation and nuclear power industries. However, experience has shown that there is a need for different types of sealing arrangements for fasteners, especially in response to specific operational requirements.
It is therefore an object of the present invention to provide high temperature seals for hydraulic assemblies such as fasteners which have improved sealing characteristics able to tolerate extreme factors such as high pressures and/or high temperatures. It is a further object of the invention to provide seals which achieve a greater extended service life under such adverse conditions or at least provide an alternative to prior art seals.